1. Field of the Invention
The present invention relates to a method of measuring a small amount of impurities and an impurity measuring device.
2. Description of the Related Art
In a process of manufacturing a semiconductor apparatus, metallic impurities,.such as Na, K and Fe, are undesirably mixed to a thin film such as a polycrystal silicon film which is formed in or on a semiconductor substrate. It is generally known that if a very small amount of such metallic impurities are mixed, a bad influence is exerted on an electric characteristic of the semiconductor apparatus. For this reason, in order to improve the electric characteristic of the semiconductor apparatus, it is necessary to accurately grasp an amount of the mixed metallic impurities in each manufacturing steps and take measures to suppress the mixing of impurities.
The following explains a conventional device and method of measuring a n amount of metallic impurities existing in and on a thin film which is formed in or on a semiconductor substrate or on a surface of a semiconductor substrate with reference to FIGS. 4, 5A and 5B.
FIG. 4 is a perspective view which shows an arrangement of a conventional impurity measuring device, FIG. 5A is a top view of the conventional impurity measuring device of FIG. 4, and FIG. 5B is a cross sectional view taken along line VB--VB of the impurity measuring device. The conventional impurity measuring device is composed of a substrate holding stand 1, a frame 2 and four screws 3. The substrate holding stand 1 has a concave section for holding a semiconductor substrate 10, and a depth of the concave section is smaller than a thickness of the semiconductor substrate 10. The frame 2 has an opening which penetrate the frame 2 at its center portion, and a diameter of the opening is smaller than a diameter of the semiconductor substrate 10. Moreover, the substrate holding stand 1 and the frame 2 respectively have holes for the screws 3 at each corner. The holding stand 1, the frame 2 and the four screws 3 are made of, for example, Teflon.
In the conventional impurity measuring method, first, the semiconductor substrate 10 on which a film is not formed or the semiconductor substrate 10 on which a thin film is formed, is placed on the concave section of the holding stand 1, then the frame 2 is placed on the semiconductor substrate 10, and the corners of the holding stand 1 are fitted on the corresponding corners of the frame 2 so that the corners are fastened with the screws 3. As a result, the frame 2 is fixed tight on the semiconductor substrate 10. Next, a solution 4, which dissolves the semiconductor substrate 10 or the thin film on the semiconductor substrate, is poured into a cavity formed by the opening of the frame 2 and the substrate 10, and the semiconductor substrate 10 or the thin film of the semiconductor substrate are dissolved by the solution 4. Then, an impurity, which exists in the thin film and on the surface of the thin film formed in or on the semiconductor substrate 10 or on the surface of the semiconductor substrate 10, exists in the solution 4 as ion. Thereafter, the impurity contained in the solution 4 is measured by, for example, an atomic absorption measuring device.
At this time, the higher the concentration of the impurity in the solution is, the more the sensitivity of the measurement is improved. Therefore, in the case where an extremely small amount of impurities are measured, it is necessary to decrease an amount of the solution to minimum amount. However, in the above method, since a comparatively large amount of the solution 4, namely, at least about 10 ml is required for dissolving the substrate 10, the impurity concentration becomes low and thus the sensitivity is not so high.
For this reason, in order to improve the sensitivity of the measurement, after the semiconductor substrate or the thin film on the semiconductor substrate is dissolved, a method of concentrating the solution is used for measuring impurities contained in the solution. However, this method requires a long time for the concentration, and the solution might be possibly contaminated by more impurities during the concentrating process.
Furthermore, in the case where impurities in a silicon substrate are measured, the silicon substrate is dissolved by a volatile solution such as fluoric acid, and the dissolved silicon substrate is changed into fluoride silicon having volatility. As a result, the silicon can be easily removed, and thus the impurities can be measured easily. However, since it is difficult to remove materials such as Al and Ti other than the silicon, a very small impurities contained in an Al thin film and a Ti thin film could not be measured.
In addition, as the diameter of the semiconductor substrate is increased, a larger amount of solution and longer time are required for dissolving the semiconductor substrate or the thin film on the semiconductor substrate. Moreover, there arises a problem that time required for concentrating a lot of solutions becomes longer.
As mentioned above in the conventional impurity measuring method and impurity measuring device, since it is necessary to concentrate the solution in order to measure a very small of impurities, a longer time is required for the concentration, and the solution might be contaminated by impurities during the concentration. As another problem, since materials such as Al and Ti other than silicon are not removed, impurities contained in the Al thin film and the Ti thin film cannot be measured.